FIG. 1 illustrates a block diagram of a magneto-optical-current sensor disclosed in patent application Ser. No. 212,579, filed June 28, 1988, now U.S. Pat. No. 4,947,107, which is assigned to the Assignee of the present invention. The system disclosed in the aforementioned application has a light source 10 which produces a beam of light 12 which is imaged upon a plane polarizer 14 which polarizes the beam of light into a first plane of polarization. The output beam of light 16 from the plane polarizer 14 is imaged upon an optical sensor 18 which exhibits the Faraday effect. The optical sensor 18 functions to rotate the plane of polarization of the incident beam 16 by an amount proportional to a magnitude of current flowing in a electrical conductor (not illustrated) which produces a magnetic field coupled to the optical sensor 18. The output beam 20 has a plane of polarization which is rotated by an angle which is directly proportional to the magnitude of the current flowing in the conductor. The light beam 20 is coupled to an analyzer 22 which includes an analyzing polarizing beam splitter (not illustrated) which divides the incident light of beam 20 into first and second beams 24. The plane of polarization of the polarizing beam splitter within the analyzer 22 is rotated 45.degree. with respect to the plane of polarization of the plane polarizer 14. In the absence of current flow in the conductor, the magneto-optic current sensor 18 does not cause any rotation of the incident beam 16 which results in components being outputted of equal magnitude from the analyzer in the aforementioned first and second beams 24. The first and second beams 24 of the analyzer are applied to a nulling and output circuit 26 which photodetects the light beams and arithmetically processes the resultant signals to produce an output 28 which is proportional to the magnitude of the current flowing in the conductor. The nulling and output circuit feeds back a signal which drives a coil (not illustrated) wrapped around a magneto-optic current sensor 18. The flow of current through the coil produces a magnetic field which opposes and cancels the magnetic field produced by the current flow in the conductor. As a result, the rotation of the plane of polarization of the beam 20 is nulled to the plane of polarization of the incident light beam 16. The aforementioned system has disadvantages in applications such as airframes where it is desirable to measure current flow in remote parts of the airframe. Application of the system of FIG. 1 to an airframe for measuring current requires an electrical conductor 30 extending from the nulling and output circuit 26 directly to the point of sensing around the current sensor 18. The electrical wire 30 is susceptible to electromagnetic interference especially where a long length of wire is required to run from the nulling and output circuit 26 to the location of the magneto-optic current sensor 18.
Furthermore, other systems are known which utilize nulling to measure current flow magneto-optically. See U.S. Pat. Nos. 3,419,802, 3,502,978, 3,581,202, 4,516,073 and 4,370,612.
U.S. Pat. No. 4,539,519 discloses a magneto-optic current measuring system which feeds back a signal for nulling the effects of birefringence in an optical fiber conducting light which is magnetically coupled to the magnetic field produced by a current being measured. The current measuring system of the '519 patent does not utilize nulled detection for measuring the magnitude of the current.